PENSION PLANS AND OTHER POST-RETIREMENT BENEFITS - Components of net periodic pension cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Defined Benefit Plans
Components of net periodic pension costs
Interest cost	$ 5,681	$ 6,520
Expected return on plan assets	(9,263)	(8,645)
Net amortization loss (gain)	557	311
Net prior service cost (credit) amortization	30	30
Net periodic pension cost (benefit)	(2,995)	(1,784)
Post-retirement Benefit Obligations
Components of net periodic pension costs
Service cost	222	258
Interest cost	657	884
Expected return on plan assets	(50)	(46)
Net amortization loss (gain)	(142)	(299)
Net prior service cost (credit) amortization	(225)	412
Net periodic pension cost (benefit)	$ 462	$ 1,209